This invention relates to separation of mixtures by differential adsorption, and more particularly to improved adsorption systems in which adsorbent bodies comprising a nucleophilic material, such as carbon, silica, alumina or a polymer having a hydrocarbon moiety, are bonded to a substrate via a siloxane polymer. Adsorbent bodies comprising carbon are preferably bonded to the siloxane polymer via a direct C--Si bond.
Various forms of carbon serve as effective media for chromatographic columns and solid phase extraction devices. Conventionally, where carbon has served as the stationary phase of a chromatographic column, the carbon is present in the column in the form of packing. However, when a sample matrix fluid is passed as the mobile phase through a column containing a fine carbon packing, the pressure drop through the column is generally high. Moreover, there is a tendency for the carbon to become entrained in the matrix fluid. Both entrainment and pressure drop can be minimized by using a granular carbon of relatively large particle size, but such coarse packing has a low adsorptive surface area per unit of column length and/or low resolution of the introduced analytes.
Adsorbent bodies of other nucleophilic materials, such as alumina, silica, zeolite and porous polymers are preferably also of small particle size; and packed columns containing such adsorbent materials typically present the same pressure drop and entrainment problems as packed carbon columns.
By binding a relatively fine carbon or other adsorbent material to the internal wall of a tube, a chromatographic apparatus may be provided which presents a substantial adsorptive surface area, yet may be operated at low pressure drop. By binding a high surface area adsorbent to the interior wall of a column, a column of a given diameter can accommodate a given flow of sample matrix at a much lower pressure drop than a column that is packed with an adsorbent of comparable particle size, and comparable adsorptive surface area per unit of column length. Consequently, at comparable pressure drop, the coated wall column can be of much smaller diameter than the packed column.
It has been found that much enhanced separation is achieved in columns of very small diameter, especially those in the capillary range, since axial backmixing is greatly minimized. Various adsorbent materials are used in such columns, including porous silica, zeolite molecular sieves and various forms of carbon. Such columns are generally referred to as "porous layer open tubular" (or PLOT) columns. Where a liquid stationary phase is coated over a porous support, the column may be referred to as "support coated open tubular" (or SCOT). If the adsorbent material is carbon or graphite, capillary columns of this type are conventionally referred to "carbon layer open tubular" (CLOT) or "graphite layer open tubular" (GLOT). All operate at low pressure drop by providing an essentially unobstructed path for the mobile phase to flow over the porous adsorbent or support.
Activated charcoal particles have been coated onto the interior wall of a glass column by using high molecular weight waxes or organic liquids as binding agents. For example, Vidal-Madjar et al., "Fast Analysis of Geometrical Isomers of Complex Compounds by Gas-Solid Chromatography," Gas Chromatograph, Elsevier (Amsterdam 1970), pp. 20-23, describe the preparation of a capillary column containing graphitized carbon adhered to the interior column wall using a styrene polymer as a form of adhesive. Another common coating material used for this purpose is polyethylene glycol, typically modified to contain a carboxyl functionality for greater polarity. Carbon, zeolite, alumina and silica adsorbent particles have been adhered to the interior wall of a PLOT column using such coatings. However, the coating materials of the above described type bind to adsorbent bodies via Van der Waals forces only. Consequently, the column must be operated below the temperature at which the wax, polystyrene, polyethylene glycol or other bonding agent softens, or else the bond between carbon and the glass loses strength sufficient to resist entrainment. Typically, this means that the column cannot be operated above a limit of about 115.degree. C. Even at temperatures below such limit, the carbon or other adsorbent particles have a tendency to break free into the matrix phase as it moves through the column. Moreover, the adsorbent particles are especially prone to entrainment in rinsing liquids or purging gases, which typically move at higher velocities than the sample matrix.
Solid phase extraction devices are advantageously constructed of a single fiber that may be injected into a sample via the cannula of a syringe; or bundle or cluster of fibers, for example, fibers arrayed in parallel in a manner similar to the bristles of a brush. Particles of adsorptive material are coated onto the fiber. In other devices known to the art, the adsorbent particles are enmeshed or otherwise mechanically trapped in a woven or blown fiber fabric, typically formed as or cut into a disk. Immersion of the device in a liquid sample matrix presents a large area of adsorptive surface on which a solute may be extracted from the matrix. A type of solid phase extraction device, commonly referred to as a "denuder," is used for selective removal of a component of a gas sample, such as an air sample. The adsorptive agent of a said phase extraction device is typically coated onto the surface of glass fibers in a manner comparable to the attachment of adsorptive particles to the inside wall of a chromatographic column as described above. Thus, activated carbon and other solid phase extraction devices known to the art have suffered from the same disadvantages as their chromatographic counterparts. Operating temperatures are limited; and the adsorptive particles are rather readily wiped off the fiber surfaces by contact with a sample matrix or rinsing liquid.
The tubular walls of chromatographic columns and the fibers of solid phase extraction devices are both preferably constituted of glass. However, a variety of metals and plastic materials may also be suitable, especially as materials for the walls of a chromatographic column.
A need has existed for an improved means of adhering carbon, zeolite, alumina, silica and adsorptive organic polymer particles to the tubular walls of chromatographic columns and the exterior surfaces of the fibers of solid phase extraction devices. A particular need has existed for adhesives which bond effectively both the materials of an adsorbent body, such as carbon, zeolite, alumina, silica, and porous organic polymers, and to the materials of the column wall or fiber surface, such as glass, metal or plastic. Since glass is ordinarily the preferred material of construction for both column tubes and the fibers of extraction devices, there has been a particular need for a better form of adhesion of carbon to glass. It has been known that C--OH groups at the surfaces of carbon particles react with silanizing agents, such as dimethyldichlorosilane, to produce an Si--O--C bond. However, this form of bonding has not heretofore been recognized as effective for attaching adsorbent carbon or adsorptive polymer particles to either the internal wall of a glass chromatographic column or the exterior surface of the glass fibers of a solid phase extraction device. It has also been known in the art that direct C--Si bonds can be formed at high temperature by solid phase reaction between carbon and silicon, resulting in the formation of silicon carbide, which is well known for use as an abrasive but has not had application to the field of chemical separations.
Recently, the literature has reported hydrosilylation reactions of buckminsterfullerene, C.sub.60, with various silanes. West et al., "C.sub.60 -Siloxane Polymers from Hydrosilylation Reactions," Polymer Preprints, Vol. 34, No. 1 (1993) describes the reaction of C.sub.60 with methyldimethoxysilane, H(Me.sub.2 SiO).sub.3 SiMe.sub.2 H, and Me.sub.3 SiO-(-HSiMeO).sub.n -(n-OctSiMeO-).sub.3n -SiMe.sub.3 (DP=30), respectively. The authors describe the resulting products as C.sub.60 molecules surrounded and encapsulated by the bound polysiloxane. However, no utility is suggested for these compositions.
U.S. Pat. No. 5,308,481 describes a polymeric or siliceous support particle suitable for use in chromatographic separations having a buckminsterfullerene covalently bonded thereto. In bonding the buckminsterfullerene to a siliceous particle, the surface of the silica may be modified by bonding a silane thereto, heating to polymerize the resulting silicon layer and attaching the buckminsterfullerene to the resultant silicone polymer via a functionality on the fullerene. For example, the '481 patent describes reacting a fullerene with a diphenylmethyl silane functionalized silica gel in the presence of aluminum chloride to produce a structure in which the fullerene is bound to the silica through an O--SiR--C.sub.6 H.sub.4 -- linkage.
A general need has existed in the art for securely adhering carbon or other granular or particulate bodies to the surface of a substrate. By way of particular example, it may be desirable to adhere catalyst bodies to the wall of reactor vessel, exhaust converter or other tubular fluid flow conduit. Further applications exist in which it may be desirable to adhere fibrous bodies to a substrate surface.